The fumigation technique is today widely used for the disinfection of soils or plant substrates, in particular those intended for intensive agriculture and in particular those intended for tree cultivation, for horticulture and for market gardening.
This fumigation technique uses at least one fumigant, generally a volatile pesticidal compound, which is introduced into the soil or the substrate to be treated, according to various techniques known to those skilled in the art, for example using colters, or nozzles for injection into the soil, or else by drip. This fumigation technique also comprises the use of at least one fumigant in the form of a gas or fog, above the soil or plant substrate to be treated.
The fumigant diffuses into the soil or more generally into the substrate to be disinfected, but also rises back up to the surface and can be dissipated into the atmosphere. Large amounts of fumigant can thus be lost, leading to a loss of efficiency of the product used. In addition, the fumigant thus dissipated into the atmosphere can be a nuisance or even toxic for farmers and the entourage in the immediate proximity of the crops and fields treated.
In order to overcome this drawback, it is common practice to cover the soil treated by fumigation with a polymer film which is impermeable to the vapors of the fumigant, as described, for example, in EP-A1-0 766 913. This gas-impermeable, plastic sheet prevents said fumigant from dispersing in the air above the soil or the substrate to be treated. In this way, there is a space between the soil or substrate and the polymer film, in which the fumigant vapors are concentrated, thus reinforcing the efficiency of said fumigant. Various types of films, such as polyethylene films, or films of SIF (semi-impermeable film), VIF (virtually impermeable film) or TIF (totally impermeable film) type, are today used during soil or substrate treatments by fumigation.
The films for disinfecting agricultural soils can be classified into two categories according to the duration of use:    a) Category 1: “Simple” Protection: These films are kept in place during the required duration of disinfection of the substrate to be treated, and are then removed before the substrate is used for growing. This category comprises two subcategories, depending on whether or not the films are assembled together by adhesive bonding:            i) films put in place without adhesive bonding, film on film,        ii) films of which the surface condition allows assembly of the strips by adhesive bonding in situ;            b) Category 2: Protection and Mulching: The films of this category first of all provide protection during the disinfection, and are then kept in place as mulching films.
The polymer films are advantageously placed on the soil or substrate before or after the treatment by fumigation, and left in place for the time necessary to allow effective control of nematodes, phytopathogenic fungi, weeds, harmful insects and other bacteria. After this treatment period, the duration of which greatly depends on the soils or substrates to be treated, on the climatic conditions, on the type of crop envisioned, and the like, the polymer films can be, where appropriate, removed or simply perforated, in order to allow the planting of crops.
The use of polymer films in fields has other advantages, such as increased soil temperature, in particular at the beginning of spring, fewer problems associated with the appearance of weeds, moisture content retention, a reduction in the number of certain insect pests, higher yields and a more efficient use of soil nutrients.
Most mulch films are generally black for weed killing, white for cooling, or clear for short-duration disinfections or for warming the soil. The temperature of the soil under a plastic mulch depends on the thermal properties (reflection, adsorption or transmittance) of the particular constituent material of the film, with respect to the entering solar radiation.
For example, black mulches retain moisture content and heat while at the same time preserving infestation by weeds. Black, the predominant color used in vegetable production, is an opaque absorber and a radiator. Black mulch absorbs most ultraviolet (UV), visible and infrared (IR) wavelengths of incident solar radiation and re-emits part of the absorbed energy in the form of thermal or infrared radiation. A large part of the solar energy absorbed by black plastic mulch is lost into the atmosphere by radiation and by forced convection.
On the other hand, transparent polymer films sparingly absorb solar radiation, but transmit from 85% to 95% of said radiation, with a relative transmittance which depends on the thickness and the degree of opacity of the film. The surface under these polymer mulches is generally covered with drops of condensed water. This water is transparent to the entering short-wave radiation, but is opaque to the leaving infrared thermal radiation, heat lost into the atmosphere from an uncovered soil due to infrared radiation, but which is retained by the transparent polymer mulch.
White films, for their part, can lead to a slight decrease in soil temperature compared to an uncovered soil, since they reflect, into the plant cover, most of the incident solar radiation. These mulches can be used to establish a crop when the soil temperatures are high, for instance in very sunny regions and where any reduction in soil temperature is beneficial.
There is therefore today a large amount of polymer films used in agriculture. The use of such films could be coupled with the use of fumigant, as previously indicated. However, when the polymer film is withdrawn or perforated, the fumigant still present in the form of vapors between the soil and said film escapes into the atmosphere and can thus be harmful to the environment, without mentioning the operators who are also widely exposed to said vapors of said fumigant.
In order to avoid exposure of the operators to the toxic and/or malodorous vapors of fumigants, the wearing of filtering masks or of specific respiratory apparatuses has been envisioned. Their use is, however, inconvenient and it is often noted that operators do not use these devices, which are nevertheless often necessary.
In addition, fumigants are in most cases toxic products, the inhalation of which by operators and the dissipation of which into the atmosphere should be avoided. Such is the case, for example, of methyl bromide which is today used less and less frequently because of its toxicity. Moreover, methyl bromide is now prohibited by the Montreal protocol, since this fumigant is considered to be a substance capable of destroying the stratospheric ozone layer. Patent application JP 9-263502 proposes another solution for avoiding the dispersion of methyl bromide into the atmosphere. This solution consists in using a multilayer photocatalytic film, an upper layer permeable to solar radiation, and a lower layer permeable to the fumigant used. Particles of titanium dioxide, acting as a photocatalyst, are deposited according to a “spray/coating” process on the lower layer, and then covered by lamination and sealing of the upper layer.
However, the photocatalytic films presented in said patent application dating from 1996 are not entirely satisfactory, and have not, thus far, been marketed. The preparation thereof on an industrial scale appears to be difficult to implement, and these films especially would not exhibit the mechanical properties required by the standards relating to films for the disinfection of agricultural soils (AFNOR NF T 54-195), in particular with regard to the specifications relating to delamination, tensile strength, tearing, slow perforation and bonding ability of the film.
In addition, spray-deposited titanium dioxide (TiO2) particles have the drawback of being able to be washed away by the condensation water running on or under the films. Moreover, the spray-coating technique imposes relatively low manufacturing rates, thus leading to a high final manufacturing cost for this type of structure.
More specifically, the examples of catalytic films described in patent application JP 9-263502 are films composed of a first layer of poly(vinylidene chloride) or of nylon and of a second layer of poly(ethylene) or of poly(vinyl chloride). A layer of ultrafine particles of titanium dioxide is deposited between these two layers. The whole assembly is thermosealed on at least part of the surface of the film. Not only does this film therefore appear to be very difficult to prepare, but it also requires a difficult thermosealing operation in the presence of the photocatalyst particles.
Another example presented in patent application JP 9-263502 shows a bilayer film in which the lower layer consist of poly(tetrafluoroethylene) comprising ultrafine particles of titanium dioxide (up to a quantitative ratio of 1:9). The nature of the other constituent polymer film of the bilayer is not indicated.
The compatibility of the titanium particles with the poly(tetrafluoroethylene) does not appear to be optimal (the film does not therefore appear to be very solid for the uses envisioned) since it is indicated, in another example of this same patent application, that it is preferable to use a flocculant (“bulking agent”), such as talc, aluminum hydroxide, calcium carbonate or porous silica, to obtain a better distribution of the photocatalyst particles within the polymer matrix.
Finally, the films described in patent application JP 9-263502 do not work with colored films since they do not take into account the colorants of which they are composed and which have the ability to adsorb the ultraviolet radiation required for the catalytic activity of the titanium dioxide particles.
All these examples show that the manufacturing of photocatalytic films is not easy and still remains today difficult to industrialize. There is, consequently, a need for photocatalytic films which can be used in the fumigation field and which are therefore impermeable to fumigant vapors, and which have a photocatalytic activity allowing efficient photocatalysis of fumigants. Such films should be easily industrializable and should exhibit a mechanical strength suitable for the uses envisioned, in order to be able to be easily handled and spread out over the soils or substrates to be treated by fumigation.